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IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino
acids in relation to human health. - Millward, D. J.
PLANT BASED SOURCES OF PROTEINS AND AMINO ACIDS IN
RELATION TO HUMAN HEALTH
Millward, D. J.
Director, Centre for Nutrition and Food Safety, University of Surrey, Guildford, Surrey,
UK
Keywords: Dietary proteins, plant foods, cereals, legumes, digestibility, biological
value, amino acid requirements, vegetarianism, meat, developing countries, children,
chronic diseases, cardiovascular disease, cancer, phytoestrogens
Contents
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1. Introduction
2. Protein Quantity
2.1. Digestibility
2.2. Amino Acid Composition and Biological Value
3. Difficulties in Defining Amino Acid Requirements and Protein Scoring Patterns
3.1. Identification of a Scoring Pattern for Protein Quality Evaluation
4. Adequacy of Plant Based Diets in Developing Countries for Children
5. Health Implications of Plant Protein Diets
5.1. Are Their Benefits from Lower Intakes of Indespensable Amino Acids of Plant
Based Diets?
5.2. Are There Benefits from Lower Protein Intakes of Plant Based Diets?
5.3. Increased Delivery of Nitrogen to the Lower Gut from Poorly Digested Plant
Proteins
5.4. Influences of Specific Peptide Sequences of Plant Proteins
5.5. Presence of Phytoprotectant Factors
6. Conclusions
Glossary
Bibliography
Biographical Sketch
Summary
Plant proteins in the human diet include a diverse range, which vary in terms of amino
acid composition and digestibility. They are perfectly capable of satisfying human needs
for all ages when consumed in appropriate mixtures. However, monotonous diets based
on unsupplemented cereals, and especially some root crops, may be inadequate sources
of indispensable amino acids, especially for children. While protein quantity is not an
issue, digestibility is a problem for some cereals and is generally poorly understood.
The adequacy of most common plant protein sources for children is discussed.
Calculation of an amino acid score is problematic because of the lack of agreement on
reference amino acid requirement patterns. New maintenance and age-related amino
acid requirements pattern have been suggested which appear valid when used to score
plant proteins, indicating values similar to, or less than, the biological value measured
directly in young children. When used to score plant-based diets in India, no marked
deficiencies are identified for adults, whilst for children deficiencies are only
©Encyclopedia of Life Support Systems (EOLSS)
IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino
acids in relation to human health. - Millward, D. J.
identifiable in very young children and these are relatively minor. Thus inadequate
amino acid supply is unlikely to be an issue with most cereal-based diets.
The health implications of plant protein diets is difficult to assess in relation to the
proteins per se because of the general benefit of diets rich in cereals fruit and
vegetables, but several potential factors have been proposed These include metabolic
responses to a lower essential amino acid and lower overall protein intake, but newly
emerging epidemiological data is challenging the view that low protein plant based diets
are beneficial, with data that high protein intakes reduce risk of cardiovascular disease,
hypertension and osteoporosis.
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However, with increased plasma IGF-1 bioactivity, a major risk factor for hormone
sensitive cancers, and with dietary protein intake a determinant of plasma IGF-1 levels,
the health implication of the level of dietary protein is difficult to resolve. It is most
likely that phytoprotectant factors associated with plant protein sources, especially the
isoflavones in soya, can reduce the risk of chronic disease, although little is known
about the detailed mechanisms.
1. Introduction
Of the several thousand plant species that are assumed to have contributed to the human
diet in the past, and the 150 species that have been cultivated for commercial purposes,
most of the worlds population depends on only about 20 different plant crops. These
plant protein sources provide 65% of the world supply of edible protein and are
generally divided into cereals, legumes and other vegetables, fruits and nuts, with cereal
grains providing almost half (47%) of world protein supplies. Plant protein sources in
the developed countries constitute only about a third of intake: i.e., 31% of protein
intake in the US diet and 36% in the UK, but are the major source (about 80%) in the
developing countries, of which cereals predominate (see Table 1). Of these wheat,
(43%), rice, (39%) and maize, (12%) account for the main part. Plant protein sources
can differ from animal sources in terms of digestibility, amino acid composition, the
presence of anti-nutritional factors which adversely influence digestibility and safety
and of phytoprotectant factors (such as antioxidants, phytoestrogens, etc.), which may
be advantageous by mediating disease protection. Due to this latter factor, together with
the current guidelines to reduce animal fat and limit meat intakes, an increased
consumption of plant food sources (fruit and vegetables, bread, cereals and potatoes)
has been universally proposed as part of the Healthy Diet.
Although it is frequently pointed out that plants can provide all of human protein needs,
it is nevertheless the case that the misconception persists that they are nutritionally
inferior to animal proteins. This is because of both complex social and cultural attitudes
towards meat and because of the scientific tradition of protein quality evaluation in
animals. In fact, the important nutritional question is not whether plant proteins can
completely provide for human amino acid needs, since this has been established for all
ages. Rather, the question is whether this is an easy task in practice, i.e., achievable with
relatively unsupplemented, low-cost cereal or other staple diets available to poor
developing communities or only possible with the much higher cost, carefully selected
mixed diets consumed by affluent vegetarians.
©Encyclopedia of Life Support Systems (EOLSS)
IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino acids in relation to human health. - Millward, D. J.
Protein sources
US
UK
Tunisia
Egypt
Brazil
Nepal
Bangladesh
Sierra Leone
UK omnivores
UK vegetarians
India (mean)
Tamil Nadhu
West Bengal
Protein
P:E ratio
g d-1
% energy
animal
cereal
g d-1
%
g d-1
%
Food balance sheets
73.5
65.0
24.6
21.8
52.3
57.5
22.9
25.2
19.0
20.9
55.6
61.2
12.9
14.8
59.6
68.3
27.1
41.1
22.6
34.3
7.3
14.6
34.8
69.5
4.8
11.3
33.4
78.6
6.9
20.3
17.0
50.0
Food intake data
44.1
59.4
17.3
23.3
18.1
33.6
18.7
34.8
3.4
5.5
47.8
76.7
2.4
5.3
29.0
63.6
0.8
1.5
48.0
89.9
Lysine requirements (at 65 kg body weight)
FAO/WHO 1973
FAO/WHO 1991
mean value
Safe allowance
MIT scoring pattern
Original N balance data recalculated
Pulse/
soya
g d-1
rest
lysine
g d-1
mg d-1
2.0
2.3
6.0
5.8
10.4
3.4
2.6
4.6
12.9
13.5
10.3
9.0
5.8
4.6
1.7
5.5
7598
5815
3911
3502
3918
2132
1883
1741
7.0
9.4
7.3
6.6
3.2
5.9
7.6
3.8
7.6
1.4
4824
2871
2413
2006
1869
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Country
113
91
91
87
66
50
43
34
12.1
11.0
10.9
9.4
9.3
10.2
8.4
8.0
74
54
62
46
53
14.2
12.7
11.1
9.7
8.8
Toronto Breakpoint studies:
Table 1. Protein and lysine content of diets in relation to estimates of the requirements.
©Encyclopedia of Life Support Systems (EOLSS)
780
3770
2795
4114
1950
1209
IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino
acids in relation to human health. - Millward, D. J.
2 Protein Quantity
Animal food sources are generally high protein, so that there is a clear relationship
between protein intake and the proportion of animal foods, especially meat, in the diet.
As shown in Table 1, the overall protein energy ratio of national food supply falls from
12.1% in the US to 8% in a Sub-Saharan African country, like Sierra Leone, as the
animal protein intake falls from 74g d-1 to 6.9g d-1. In the UK, the P:E ratio falls from
14.2% in omnivores to 12.7% in the small number of vegetarians, (non meat eaters)
identified in the UK food intake survey. Thus, protein intakes of vegetarians are likely
to be closer to the RNI, and some 20-30% of this, albeit small, sample were below it.
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Whether this is a problem is to some extent debatable, but probably unlikely especially
in developed countries given the overall lower morbidity and mortality of vegetarians
compared with meat eaters. Firstly with the adult protein requirement equivalent to a
P:E ratio of about 9% and 7% for the Reference Nutrient Intake (RNI) and Estimated
Average Requirement EAR, i.e., 0.75g and 0.6g protein kg-1 in adults, (calculated
assuming an energy requirement of 1.6* BMR), the lowest diets shown in Table 1 fall
between the EAR and RNI for adults. Secondly, all of the values in Table 1, for protein
as food supplies or intake, indicate P:E values higher than breast milk at 7%, since as
shown in Table 2, only a few plant staples have a lower P:E ratio than this. Indeed.
wheat and maize are “high protein foods” compared with breast milk if the energy
density issue is ignored. Furthermore on the basis of a metabolic model for the protein
requirement which includes a substantial adaptive component varying with intake (see
below), there is by definition a correlation between intake and requirement, so that low
intakes are unlikely to become associated with substantial prevalence rates of
inadequacies until they fall close to the LRNI. Most importantly, at least as far as food
supply data is concerned, wheat-based food supplies, such as Tunisia and Egypt with
only 15-20% animal protein sources, clearly supply protein at levels close to that of
predominantly animal based food supplies, as for the UK. This means that cereal based
diets, especially those based on wheat, can supply protein at levels well above the
human protein requirement. As for infants and children, the wide and successful use of
soya based infant formula is proof that plant-based diets can be adequate for infants. On
the other hand, the monotonous diets based on very low protein starchy root crops, such
as cassava, may well supply inadequate protein intakes to ensure adequate height
growth. In this context, it is interesting to note that in such cases, such as the stunted
Bundi orphanage children described in the 1970s who were fed almost exclusively on
the low protein starchy root Taro, stunting was the only observable symptom of any
nutritional adequacy. They were otherwise healthy, without overt symptoms of
Kwashiorkor, in support of the arguments that Kwashiorkor is not a protein-deficiency
disease. The issue of whether stunting in children reflects inherent inadequacies of plant
based diets as protein sources as opposed to other nutrient inadequacies is outside the
scope of this paper, but as discussed below, it has been demonstrated that young
children fed one of the hybrid varieties of maize (opaque-2(o2), sugary-2(su2) hybrid)
(see Molecular Genetic Improvement of Protein Quality in Maize) as their sole protein
and energy source (but with mineral and vitamin supplementation) grow in height and
weight at rates similar to that achieved with casein. For these reasons, and with the
nutritional adequacy question limited to the consideration of protein needs, then we can
reasonably safely conclude that with the exception of some starchy roots, plant based
©Encyclopedia of Life Support Systems (EOLSS)
IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino
acids in relation to human health. - Millward, D. J.
diets available in most parts of the world are capable of providing adequate protein for
all ages. Thus, protein quantity is unlikely to be an issue and the main question of their
nutritional adequacy as protein sources is limited to their quality, i.e., digestibility and
biological value.
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Bibliography
Anderson JJB, Anthony M, Messina M & Garner SC (1999) Effects of phyto-oestrogens on tissues.
Nutrition Research Reviews 12, 75–116. [This is a comprehensive review of the influence of soy
isoflavones on metabolism].
Bingham, S. A. (1999) High-meat diets and cancer risk. Proceedings of the Nutrition Society 58, 243248. [This paper presents one perspective on a possible meat-colon cancer link].
Cassidy A & Griffin BA (1999) Phyto-oestrogens: a potential role in the prevention of CHD?
Proceedings of the Nutrition Society 58, 193–199. [This paper examines how plant proteins can influence
heart disease with the example of soy].
FAO/WHO/UNU. (1985) Energy and protein requirements. 15. Report of a joint FAO/WHO/UNU expert
consultation. Technical report series 724. Geneva: WHO. [This report discusses how protein requirements are
defined].
FAO/WHO (1991) Protein Quality Evaluation. Report of a joint FAO/ WHO expert consultation. FAO Food
and Nutrition Paper No 51. Rome: Food and Agriculture Organization of the United Nations. [This report
discusses how protein quality is assessed].
Friedman M. (1996) Nutritional value of proteins from different food sources. A review Journal of
Agricultural & Food Chemistry 44, 6-29, [This is a comprehensive review of the nutritional value of a wide
range of plant protein sources].
Key, TJ, Davey GK & Appleby PN (1999) Health benefits of a vegetarian diet. Proceedings of the
Nutrition Society 58, 214-218 [This reviews the epidemiological evidence for health benefits of plant
based protein sources].
McCarty, M. F. (1999) Vegan proteins may reduce risk of cancer, obesity, and cardiovascular disease by
promoting increased glucagon activity. Medical Hypotheses 53, 459-485. [This is a speculative review on
how plant proteins could have health benefits].
Millward DJ. (1998a) Quality and utilization of plant proteins in human nutrition In Plant Proteins from
European Crops: Food and non-Food Applications (J Gueguen & Y Popineau Eds) Springer-Verlag Berlin
pp169-176 [This paper reviews how the nutritional value of plant proteins has been measured in human
nutrition].
Millward DJ (1999a) The nutritional value of plant based diets in relation to human amino acid and
protein requirements. Proceedings of the Nutrition Society 58, 249-260. [This paper includes a
comprehensive review of the current views of human amino acid requirements, differences in essential
amino acid content between plant and animal proteins and the nutritional value of plant compared with
animal proteins in human nutrition].
Millward DJ (1999b) Optimal intakes of dietary protein. Proceedings of the Nutrition Society 58, 403-413.
©Encyclopedia of Life Support Systems (EOLSS)
IMPACTS OF AGRICULTURE ON HUMAN HEALTH AND NUTRITION – Vol. I - Plant based sources of proteins and amino
acids in relation to human health. - Millward, D. J.
[This paper includes a review of the influence of the level of protein in the diet on health, especially bone
health kidney function, fetal development and hypertension].
Pellett P.L (1996) World essential amino acid supply with special attention to South-East Asia. Food and
Nutrition Bulletin 17, 204-234 [This paper includes a comprehensive review of the amino acid supplies
and their adequacy].
Rosegrant MW (1999) Alternative futures for world cereal and meat consumption. Proceedings of the
Nutrition Society 58, 219-234 [This paper is an interesting account of the impact of changes in meat
consumption in the developed world on poverty in the developing world].
Young, V.R. & Pellet, P.L., (1994) Plant proteins in relation to human protein and amino acid nutrition
American Journal of Clinical Nutrition 59, 1203S-1212S [This paper reviews the differences in essential
amino acid content between plant and animal proteins, and discusses the nutritional value of plant
compared with animal proteins in human nutrition].
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Biographical Sketch
D Joe Millward, B.Sc., Ph.D., D.Sc. is Professor of Nutrition and Director at the Centre for Nutrition
and Food Safety at the University of Surrey in the UK. He trained as a biochemist, obtaining his initial
degree and D Sc from the University of Wales, and PhD from the University of the West Indies. He
worked for many years with John Waterlow, initially at the Medical Research Councils Tropical
Metabolism Research Unit in Jamaica, and subsequently in the Department of Human Nutrition at the
London School of Hygiene and Tropical Medicine, as Reader in Nutritional Biochemistry. He has taught
nutrition and metabolism for over 25 years, at both undergraduate and post graduate level, and has
recently initiated the first post-graduate course in Nutritional Medicine in the UK. His research has
focused on a) the regulation of protein metabolism and turnover, and the metabolic basis of protein and
amino acid requirements, b) the nature of the influences of dietary fat on chronic disease. He advises the
FAO, and the WHO on protein requirements and protein quality. He is, or has been, an Editor of the
American Journal of Physiology, the British Journal of Nutrition, Nutrition Research Reviews, and
Clinical Science. Recent papers include Millward DJ (1999) Meat or wheat for the next millenium? Proc
Nutr Soc 58, 209-210; Millward DJ (1999) The nutritional value of plant based diets in relation to human
amino acid and protein requirements. Proc Nutr Soc 58, 249-260; Millward DJ (1999) Optimal intakes of
dietary protein. Proc Nutr Soc 58, 403-413; Millward DJ et al. (2000) Human adult protein and amino
acid requirements: [13C-1] leucine balance evaluation of the efficiency of utilization and apparent
requirements for wheat protein and lysine compared with milk protein in healthy adults. Am J Clin Nutr.
72, 112-121.
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